A recent newsletter from Vincent Flanders had a link that could turn your site grey for testing usability. Yahoo also turned their entire site grey for the day of 9/11. Here is the code that Vincent sent along with a few others I found to play with:
Grey Scale: javascript:document.body.style.filter='gray()';void(null)
Negative: javascript:document.body.style.filter='invert()';void(null)
Blur: javascript:document.body.style.filter='blur()';void(null)
X-ray: javascript:document.body.style.filter='xray()';void(null)
Flip H: javascript:document.body.style.filter='fliph()';void(null)
Flip V: javascript:document.body.style.filter='flipv()';void(null)
You can apply this to any site by copying the code to the address bar.
update these don't work very well in Firefox or CSS designed sites.